AvrPtoB

AvrPtoB is a type III effector protein produced by Pseudomonas syringae, including pathovars such as Pseudomonas syringae pv. tomato DC3000. It is delivered into plant cells through the bacterial type III secretion system and functions as part of the pathogen’s arsenal to suppress plant innate immunity and promote virulence.

Biochemical analyses show that AvrPtoB comprises two functional regions. The N-terminal region is involved in translocation efficiency and interaction with host proteins, while the C-terminal region contains a U-box type E3 ubiquitin ligase domain. The ligase activity enables AvrPtoB to ubiquitinate specific plant proteins, targeting them for proteasomal degradation or altering signaling, thereby dampening defense responses.

AvrPtoB is known to affect multiple components of plant immunity, including receptors and signaling factors that participate in PAMP-triggered immunity as well as effector-triggered immunity. By promoting degradation or dysfunction of these host proteins, AvrPtoB suppresses basal defenses and facilitates bacterial growth in susceptible plants.

In tomato, the presence of the Pto kinase and the Prf resistance protein can detect AvrPtoB in certain contexts, triggering a robust hypersensitive response. However, AvrPtoB can evade detection in other plant backgrounds, illustrating a balance between virulence and surveillance.

AvrPtoB has become a model for studying how bacterial pathogens manipulate plant ubiquitination pathways. Its dual-domain architecture exemplifies how an effector protein can combine a virulence module with an enzymatic activity to subvert host defenses.